Re: [syzbot] BUG: unable to handle kernel access to user memory in schedule_tail

[Dmitry Vyukov <dvyukov@google.com>]

On Thu, Mar 11, 2021 at 7:50 AM Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Thu, Mar 11, 2021 at 7:40 AM Alex Ghiti <alex@ghiti.fr> wrote:
> >
> > Hi Ben,
> >
> > Le 3/10/21 à 5:24 PM, Ben Dooks a écrit :
> > > On 10/03/2021 17:16, Dmitry Vyukov wrote:
> > >> On Wed, Mar 10, 2021 at 5:46 PM syzbot
> > >> <syzbot+e74b94fe601ab9552d69@syzkaller.appspotmail.com> wrote:
> > >>>
> > >>> Hello,
> > >>>
> > >>> syzbot found the following issue on:
> > >>>
> > >>> HEAD commit:    0d7588ab riscv: process: Fix no prototype for
> > >>> arch_dup_tas..
> > >>> git tree:
> > >>> git://git.kernel.org/pub/scm/linux/kernel/git/riscv/linux.git fixes
> > >>> console output: https://syzkaller.appspot.com/x/log.txt?x=1212c6e6d00000
> > >>> kernel config:
> > >>> https://syzkaller.appspot.com/x/.config?x=e3c595255fb2d136
> > >>> dashboard link:
> > >>> https://syzkaller.appspot.com/bug?extid=e74b94fe601ab9552d69
> > >>> userspace arch: riscv64
> > >>>
> > >>> Unfortunately, I don't have any reproducer for this issue yet.
> > >>>
> > >>> IMPORTANT: if you fix the issue, please add the following tag to the
> > >>> commit:
> > >>> Reported-by: syzbot+e74b94fe601ab9552d69@syzkaller.appspotmail.com
> > >>
> > >> +riscv maintainers
> > >>
> > >> This is riscv64-specific.
> > >> I've seen similar crashes in put_user in other places. It looks like
> > >> put_user crashes in the user address is not mapped/protected (?).
> > >
> > > The unmapped case should have been handled.
> > >
> > > I think this issue is that the check for user-mode access added. From
> > > what I read the code may be wrong in
> > >
> > > +    if (!user_mode(regs) && addr < TASK_SIZE &&
> > > +            unlikely(!(regs->status & SR_SUM)))
> > > +        die_kernel_fault("access to user memory without uaccess routines",
> > > +                addr, regs);
> > >
> > > I think the SR_SUM check might be wrong, as I read the standard the
> > > SR_SUM should be set to disable user-space access. So the check
> > > should be unlikely(regs->status & SR_SUM) to say access without
> > > having disabled the protection.
> >
> > The check that is done seems correct to me: "The SUM (permit Supervisor
> > User Memory access) bit modifies the privilege with which S-mode loads
> > and stores access virtual memory.  *When SUM=0, S-mode memory accesses
> > to pages that are accessible by U-mode (U=1 in Figure 4.15) will fault*.
> >   When SUM=1, these accesses are permitted.SUM  has  no  effect  when
> > page-based  virtual  memory  is  not  in  effect".
> >
> > I will try to reproduce the problem locally.
>
> Weird. It crashes with this all the time:
> https://syzkaller.appspot.com/bug?extid=e74b94fe601ab9552d69
>
> Even on trivial programs that almost don't do anything.
> Maybe it's qemu bug? Do registers look sane in the dump? That SR_SUM, etc.
>
>
> 00:13:27 executing program 1:
> openat$drirender128(0xffffffffffffff9c,
> &(0x7f0000000040)='/dev/dri/renderD128\x00', 0x0, 0x0)
>
> [  812.318182][ T4833] Unable to handle kernel access to user memory
> without uaccess routines at virtual address 00000000250b60d0
> [  812.322304][ T4833] Oops [#1]
> [  812.323196][ T4833] Modules linked in:
> [  812.324110][ T4833] CPU: 1 PID: 4833 Comm: syz-executor.1 Not
> tainted 5.12.0-rc2-syzkaller-00467-g0d7588ab9ef9 #0
> [  812.325862][ T4833] Hardware name: riscv-virtio,qemu (DT)
> [  812.327561][ T4833] epc : schedule_tail+0x72/0xb2
> [  812.328640][ T4833]  ra : schedule_tail+0x70/0xb2
> [  812.330088][ T4833] epc : ffffffe00008c8b0 ra : ffffffe00008c8ae sp
> : ffffffe0238bbec0
> [  812.331312][ T4833]  gp : ffffffe005d25378 tp : ffffffe00a275b00 t0
> : 0000000000000000
> [  812.333014][ T4833]  t1 : 0000000000000001 t2 : 00000000000f4240 s0
> : ffffffe0238bbee0
> [  812.334137][ T4833]  s1 : 00000000250b60d0 a0 : 0000000000000036 a1
> : 0000000000000003
> [  812.336063][ T4833]  a2 : 1ffffffc0cfa8b00 a3 : ffffffe0000c80cc a4
> : 7f467e72c6adf800
> [  812.337398][ T4833]  a5 : 0000000000000000 a6 : 0000000000f00000 a7
> : ffffffe0000f8c84
> [  812.339287][ T4833]  s2 : 0000000000040000 s3 : ffffffe0077a96c0 s4
> : ffffffe020e67fe0
> [  812.340658][ T4833]  s5 : 0000000000004020 s6 : ffffffe0077a9b58 s7
> : ffffffe067d74850
> [  812.342492][ T4833]  s8 : ffffffe067d73e18 s9 : 0000000000000000
> s10: ffffffe00bd72280
> [  812.343668][ T4833]  s11: 000000bd067bf638 t3 : 7f467e72c6adf800 t4
> : ffffffc403ee7fb2
> [  812.345510][ T4833]  t5 : ffffffc403ee7fba t6 : 0000000000040000
> [  812.347004][ T4833] status: 0000000000000120 badaddr:
> 00000000250b60d0 cause: 000000000000000f
> [  812.348091][ T4833] Call Trace:
> [  812.349291][ T4833] [<ffffffe00008c8b0>] schedule_tail+0x72/0xb2
> [  812.350796][ T4833] [<ffffffe000005570>] ret_from_exception+0x0/0x14
> [  812.352799][ T4833] Dumping ftrace buffer:
> [  812.354328][ T4833]    (ftrace buffer empty)
> [  812.428145][ T4833] ---[ end trace 94b077e4d677ee73 ]---
>
>
> 00:10:42 executing program 1:
> bpf$ENABLE_STATS(0x20, 0x0, 0x0)
> bpf$ENABLE_STATS(0x20, 0x0, 0x0)
>
> [  646.536862][ T5163] loop0: detected capacity change from 0 to 1
> [  646.566730][ T5165] Unable to handle kernel access to user memory
> without uaccess routines at virtual address 00000000032f80d0
> [  646.586024][ T5165] Oops [#1]
> [  646.586640][ T5165] Modules linked in:
> [  646.587350][ T5165] CPU: 1 PID: 5165 Comm: syz-executor.1 Not
> tainted 5.12.0-rc2-syzkaller-00467-g0d7588ab9ef9 #0
> [  646.588209][ T5165] Hardware name: riscv-virtio,qemu (DT)
> [  646.589019][ T5165] epc : schedule_tail+0x72/0xb2
> [  646.589811][ T5165]  ra : schedule_tail+0x70/0xb2
> [  646.590435][ T5165] epc : ffffffe00008c8b0 ra : ffffffe00008c8ae sp
> : ffffffe008013ec0
> [  646.591142][ T5165]  gp : ffffffe005d25378 tp : ffffffe007634440 t0
> : 0000000000000000
> [  646.591836][ T5165]  t1 : 0000000000000001 t2 : 0000000000000008 s0
> : ffffffe008013ee0
> [  646.592509][ T5165]  s1 : 00000000032f80d0 a0 : 0000000000000004 a1
> : 0000000000000003
> [  646.593188][ T5165]  a2 : 1ffffffc0cfac500 a3 : ffffffe0000c80cc a4
> : 8d229faaffda9500
> [  646.593878][ T5165]  a5 : 0000000000000000 a6 : 0000000000f00000 a7
> : ffffffe000082eba
> [  646.594552][ T5165]  s2 : 0000000000040000 s3 : ffffffe00c82c440 s4
> : ffffffe00e61ffe0
> [  646.595253][ T5165]  s5 : 0000000000004000 s6 : ffffffe067d57e00 s7
> : ffffffe067d57850
> [  646.595938][ T5165]  s8 : ffffffe067d56e18 s9 : ffffffe067d57e00
> s10: ffffffe00c82c878
> [  646.596627][ T5165]  s11: 000000967ba7a1cc t3 : 8d229faaffda9500 t4
> : ffffffc4011bc79b
> [  646.597319][ T5165]  t5 : ffffffc4011bc79d t6 : ffffffe008de3ce8
> [  646.597909][ T5165] status: 0000000000000120 badaddr:
> 00000000032f80d0 cause: 000000000000000f
> [  646.598682][ T5165] Call Trace:
> [  646.599294][ T5165] [<ffffffe00008c8b0>] schedule_tail+0x72/0xb2
> [  646.600115][ T5165] [<ffffffe000005570>] ret_from_exception+0x0/0x14
> [  646.601333][ T5165] Dumping ftrace buffer:
> [  646.602322][ T5165]    (ftrace buffer empty)
> [  646.663691][ T5165] ---[ end trace e7b7847ce74cdfca ]---

Is it reasonable that schedule_tail is called from ret_from_exception?
Maybe the issue is in ret_from_exception? I see it does something with
registers.



> > Thanks,
> >
> > Alex
> >
> > >
> > > Without this, you can end up with an infinite loop in the fault handler.
> > >
> > >>
> > >>> Unable to handle kernel access to user memory without uaccess
> > >>> routines at virtual address 000000002749f0d0
> > >>> Oops [#1]
> > >>> Modules linked in:
> > >>> CPU: 1 PID: 4875 Comm: syz-executor.0 Not tainted
> > >>> 5.12.0-rc2-syzkaller-00467-g0d7588ab9ef9 #0
> > >>> Hardware name: riscv-virtio,qemu (DT)
> > >>> epc : schedule_tail+0x72/0xb2 kernel/sched/core.c:4264
> > >>>   ra : task_pid_vnr include/linux/sched.h:1421 [inline]
> > >>>   ra : schedule_tail+0x70/0xb2 kernel/sched/core.c:4264
> > >>> epc : ffffffe00008c8b0 ra : ffffffe00008c8ae sp : ffffffe025d17ec0
> > >>>   gp : ffffffe005d25378 tp : ffffffe00f0d0000 t0 : 0000000000000000
> > >>>   t1 : 0000000000000001 t2 : 00000000000f4240 s0 : ffffffe025d17ee0
> > >>>   s1 : 000000002749f0d0 a0 : 000000000000002a a1 : 0000000000000003
> > >>>   a2 : 1ffffffc0cfac500 a3 : ffffffe0000c80cc a4 : 5ae9db91c19bbe00
> > >>>   a5 : 0000000000000000 a6 : 0000000000f00000 a7 : ffffffe000082eba
> > >>>   s2 : 0000000000040000 s3 : ffffffe00eef96c0 s4 : ffffffe022c77fe0
> > >>>   s5 : 0000000000004000 s6 : ffffffe067d74e00 s7 : ffffffe067d74850
> > >>>   s8 : ffffffe067d73e18 s9 : ffffffe067d74e00 s10: ffffffe00eef96e8
> > >>>   s11: 000000ae6cdf8368 t3 : 5ae9db91c19bbe00 t4 : ffffffc4043cafb2
> > >>>   t5 : ffffffc4043cafba t6 : 0000000000040000
> > >>> status: 0000000000000120 badaddr: 000000002749f0d0 cause:
> > >>> 000000000000000f
> > >>> Call Trace:
> > >>> [<ffffffe00008c8b0>] schedule_tail+0x72/0xb2 kernel/sched/core.c:4264
> > >>> [<ffffffe000005570>] ret_from_exception+0x0/0x14
> > >>> Dumping ftrace buffer:
> > >>>     (ftrace buffer empty)
> > >>> ---[ end trace b5f8f9231dc87dda ]---
> > >>>
> > >>>
> > >>> ---
> > >>> This report is generated by a bot. It may contain errors.
> > >>> See https://goo.gl/tpsmEJ for more information about syzbot.
> > >>> syzbot engineers can be reached at syzkaller@googlegroups.com.
> > >>>
> > >>> syzbot will keep track of this issue. See:
> > >>> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> > >>>
> > >>> --
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> > >>> To view this discussion on the web visit
> > >>> https://groups.google.com/d/msgid/syzkaller-bugs/000000000000b74f1b05bd316729%40google.com.
> > >>>
> > >>
> > >> _______________________________________________
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> > >>
> > >
> > >